Gardening Columns

Wooly Adelgids Threaten Our Beloved Hemlocks

First published on garden.org on August 3, 2006, by Suzanne DeJohn

Since the 1920s when they were first inadvertently introduced to the U.S., the tiny hemlock wooly adelgid (HWA for short) has been slowly but steadily multiplying and spreading. These pinhead-sized insects feed on hemlock trees, damaging the foliage and buds, and the resulting defoliation can kill a majestic, 175-foot-tall, 500-year-old hemlock tree in just a few years. If their spread remains unchecked, the havoc these insects wreak on hemlocks will rival that of the blight that killed off our chestnut trees or the Dutch elm disease that decimated America's favorite street tree.

There is some hope, however, and it comes in the form of another tiny insect, Sasajiscymnus tsugae, a poppy seed-sized predatory beetle that that preys on the adelgids. More on that and other control options later -- first let's take a closer look at the players in this drama.

An Adelgid Primer
The hemlock wooly adelgid is an aphidlike insect that colonizes at the base of hemlock needles, sucking the sap and in the process injecting toxic saliva, causing the trees' needles to die. Although individual insects are small -- 1/32 inch long -- and hard to see with the naked eye, the insects cover themselves with a protective white, cottony covering that makes them more visible.

In a span of 30 years or so, the adelgid spread from its original point of introduction in the Pacific Northwest -- where it arrived accidentally, probably in shipments from Japan -- across the country to the eastern U.S. (It's likely a second inadvertent introduction was responsible for much of the eastern infestation.) Lacking natural enemies in North America, HWAs spread unchecked via wind, mammals, birds, and human activity, and the results have been devastating. In some parts of southern New England and the mid-Atlantic, HWAs have killed 80% of the hemlock trees. And now they're moving south.

In the past decade, they've moved into Tennessee and the Carolinas, where hemlocks are often the dominant evergreen species, especially in the mountains. Smoky Mountains National Park contains between 8,500 and 18,000 acres of hemlock-dominated forest.

HWAs are prolific reproducers. They are parthenogenic; reproduction does not require fertilization. All adults are female, and each adult can produce up to 300 eggs in her lifetime. There are two generations per year, which means that a single adelgid can in theory produce 90,000 offspring in one year. (300 eggs = 300 offspring, each of which can produce 300 offspring of her own -- 300 times 300 equals 90,000 -- all in one year.) It's easy to see how populations have exploded.

Hemlock's Ecological Niche
Hemlock-dominated forests constitute an ecologically unique environment. Large stands of these long-lived conifers create the cool, damp habitat favored by a host of plant and animal species, some of which are found only in this environment. Hemlock-bordered streams are crucial habitat for native trout, providing the shady, cool conditions these fish demand. A recent study showed that brook trout populations were three times greater in hemlock-shaded streams than in streams bordered solely by hardwoods.

Control Options
What can we do to save our hemlocks? There are two options for controlling HWA: insecticides and biological controls.

Insecticides. Large-scale insecticide applications are impractical, but they are an option for limited use on high value trees on public lands and in home landscapes. Insecticide treatments are not preventative, so they are only an option when you have positively identified a HWA infestation.

Horticultural oil and insecticidal soap sprays have proven effective in managing HWA when applied properly. These contact insecticides must be applied so they thoroughly cover the foliage, top and bottom. They must be applied when the insect is not covered by its protective wooly coating: in spring after egg hatch, and in September/October. Homeowners may be able to apply these sprays to small trees, but large trees will require professional application.

Imidacloprid is being used as a systemic insecticide to control HWA. It can be injected into the trunk, applied as a soil drench, or injected into the soil. The insecticide is dispersed throughout the tree by the movement of sap, and the adelgid as it feeds. Soil drenches and soil injections pose greater risks of environmental contamination than trunk injections, especially if the trees are located near waterways. These treatments should be applied in spring or fall and can protect a tree for up to 3 years. Consult a certified aborist to discuss the options.

When using any pesticide, including horticultural oil and insecticidal soap, always follow pesticide label directions to the letter.

Biological control. The best hope to date for large-scale control of HWA comes in a tiny package: Sasajiscymnus tsugae. Like the adelgid, this beetle is native to Japan where it keeps adelgid populations in check. The National Forest Service is breeding and releasing these beetles in the hopes that they'll provide similar control here in the U.S. Although introducing yet another exotic insect always poses some risk, according to the Forest Service, "The beetle has shown no undesirable traits that would cause it to be a nuisance or otherwise a poor candidate for release." These predatory beetles are appropriate for large tracts of hemlock and are not available to homeowners.

If you have hemlock trees in your landscape, inspect them regularly for adelgids. Early detection and control is your best option. Once the tree begins to defoliate, it is more likely to succumb. And let's hope that little Sasajiscymnus tsugae is up to the job of controlling these pests.